1. Field of the Invention
The present invention relates to Wavelength-Division Multiplexing (WDM) networks. In particular, the present invention relates to optical network structures that perform multiplexing and switching of WDM signals.
2. Description of the Related Art
In recent years, communication technology has rapidly and significantly improved. Today, much information is transmitted via optical fibers which allow the transport of information at high data rates. Optical fibers also offer much higher bandwidth than conventional coaxial communications. Nevertheless, an increased data transmission through communication networks places an increasing demand on communications link bandwidth. WDM is an attractive technique for increasing the aggregate capacity of optical networks. In a wavelength division multiplexed optical communication system, several different channels at different optical wavelengths are combined into a fiber to allow for a more efficient use of its bandwidth.
Initially, the WDM channels were multiplexed and coupled into an optical fiber at the transmitting end, transmitted to a receiver end using the same optical path, and then de-multiplexed at the receiver end. This was commonly referred to as a point-to-point WDM transmission. In case of point-to-point WDM transmission, the optical network nodes only served as conduits of the transmitted data without any advanced functionality.
Subsequently, WDM networks evolved from simple point-to-point transmission to more complicated multi-user networks with arbitrary physical topologies. In case of an optical network with arbitrary physical topology, different channels may follow different optical paths in the network. There are several classes of such networks: wavelength-routed, static optical and linear optical networks. A class of networks in which optical signals are routed to different optical paths based on their wavelengths are known as wavelength routed networks. The specific optical paths for each channel are determined by a routing algorithm. The routing algorithm is designed based on a specified criteria, e.g., the shortest path between a source and a destination. The optical network nodes in this class are capable of routing signals to different paths depending on their wavelengths. In static optical networks, the star couplers are used as the optical network nodes which are capable of broadcasting an input signal from any source to all destinations in the network. In linear optical networks, even more complex components are used because the optical network nodes are required both to broadcast and to route different channels. For this reason, there have been attempts to provide the network nodes that could perform specific multiplexing, filtering, routing or switching functions.
For example, in Y. Tachikawa et al., Arrayed-Waveguide Grating Multiplexers with loop-back optical paths and its applications, J. Lightwave Tech., Vol. 14, pp. 977-84 (1996), a static add/drop multiplexer which uses an N×N-array waveguide grading multiplexer combined with N-2 loop-back fibers is illustrated.
Another example is disclosed in O. Ishida et al., Digitally Tunable Optical Filters using Array-Waveguide Grating (AWG) Multiplexers and Optical Switches, J. Lightwave Tech. Vol. 15, pp. 321-27 (1997). Ishida discloses a digital tunable optical filter which uses an N×N-arrayed waveguide grating multiplexer combined with a number of 1×2 switch elements. In this example, the number of 1×2 switch elements is equal to the square root of N.
In A. A. M. Staring et al., Phased-Array-Based Photonic Integrated Circuits for Wavelength Division Multiplexing Applications, ICICE Trans. Electron., Vol. E80-C, pp. 646-653 (1997), an integrated four-channel add/drop multiplexer is illustrated. The add/drop multiplexer in this example uses a 5×5-phased array (PHASAR) multiplexer and four Mach-Zehnder interferometer (MZI) switches. A wavelength selective optical switching is accomplished by using a wavelength converter and a 5×5 PHASAR. The 5×5 PHASAR is used as a static router. The wavelength converter, in this example, has an Erbium-doped fiber amplifier (EDFA), a distributed Bragg reflector (DBR) laser, and a 3 dB coupler.
The evolution of WDM networks has imposed a functionality burden on the optical network nodes (ONN). It is desired that the signal be kept in an optical form as it passes through the network because it increases the network speed. In other words, it is desirable to preserve network transparency. However, this means that the ONNs have to perform many functions in the physical (optical) layer instead of the electronic (logical) layer.
Unfortunately, each of the multi-function transparent optical network nodes known hereto suffer from a common drawback in that they combine individual components, i.e. switches, multiplexers, filters and routers, in a conventional way to perform a specific routing and switching functions. Hence, their functionality density, i.e., the number of functions divided by the number of components used is low. Accordingly, there remains a need for a photonic WDM component with high functionality density that can couple a single-mode and a multi-mode waveguides and can combine a plurality of functions.